All internal-combustion engines containing pistons exhibit at least one crankshaft. The crankshaft constitutes the crank of a connecting rod/crank system, which allows the reciprocating motion of the piston to be transformed into the rotary motion of the engine flywheel.
In order to ensure the reliability of the link between the crankshaft at a crankpin and the corresponding connecting rod, it is necessary to lubricate this link. The lubrication consists of bringing oil under pressure into this link.
More precisely, the principal oil line of the engine feeds shaft-line/journal bearings disposed between the engine block and the crankshaft journals. These bearings include a groove in which the oil under pressure can be stored. A bore in the crankshaft going from a journal to a crankpin allows oil feed at the crankpin of the link between the crankshaft and the corresponding connecting rod. This type of architecture is known, for example, from U.S. Pat. No. 2,955,675.
The architecture of these feed lines must, in addition, meet certain imperative constraints. The bore for a journal to feed a crankpin should affect the mechanical resistance of this arm as little as possible. In order to do this, it must pass roughly to the center of the arm connecting the journal to the crankpin, which results, in particular, in a central passage at a median plane roughly equidistant from the journal axis and the crankpin. In addition, the outlet of the lubrication line at a crankpin must be executed outside zones of high stress to which the crankpin is subjected, or the oil film providing the lubrication would be disrupted and the lubrication would be less consistent in those zones where it is essential.
In order to improve the lubrication, it is known to execute a diametric bore in the journal. It involves a straight bore perpendicular to the axis of crankshaft rotation and intercepting the axis of rotation. Thus, this diametric bore is executed along a diameter of the plane of a transverse median section of the journal, orthogonally to the crankshaft axis of rotation. This diametric bore can in addition emerge at the same place as the bore between the journal and the crankpin. The diametric bore offers two inlets in the crankshaft for the oil under pressure and facilitates the circulation of the oil. The absence of such a bore can lead to discontinuous oil feed for the crankpins during an engine cycle, over about 180° of crankshaft rotation, because the oil cushion formed at the journal is generally only achievable in the upper part of the journal (cushion formed in the upper half-collar of the journal), and this is for reasons of wear and mechanical performance of the lower half-collar of the cushion.
It is in addition known, and implemented in some applications, to feed lubricating oil to two crankpins from a single journal. This greatly simplifies the lubricating system, facilitates its execution, and reduces the costs of implementation. The diametric bore then intercepts the two bores connecting the journal to the two crankpins it is feeding. The diametric bore specifically allows feed of the two crankpins to be obtained over 360° of crankshaft rotation.
Thus, the system for greasing or lubricating the crankshaft as known in prior art poses no particular problem for the majority of engines. However, this architecture, in which two crankpins are fed by one journal exhibiting a diametric bore, is no longer applicable in certain cases, particularly, for example, under certain conditions of angular shift between the two crankpins being fed and which are associated with small crankpin or journal diameters and a greater crank radius. The situation can occur more commonly, for instance, in the case of a three-cylinder engine.
In these cases, no geometric construction can guarantee, simultaneously:                a position of oil feed lines at the crankpins that is compatible with correct oil feed outside zones of high crankpin stress.        passage of the bore into a zone of the crankshaft arms that does not significantly weaken the crankshaft, specifically passing roughly to the center of the arm connecting the crankpin and the journal.        a diametric bore of the journal joining the two bores coming from the crankpins.        
Within the scope of engines exhibiting small-diameter crankpins and/or journals with an unfavorable angular offset of two crankpins, it is therefore necessary, according to the architectures known in prior art, to modify the architecture of the lubrication system in the crankshaft either/or:                by eliminating the diametric bore of the journal, but then the oil feed for the connecting rods is affected. Feed only occurs during part of the cycle and is therefore discontinuous. This drawback makes the absence of a diametric bore rare in the case in which two crankpins are fed from a single journal,        by adopting an architecture in which each journal feeds a single crankpin, which presents drawbacks in terms of complexity and cost of execution, but also brings about an increase in leakage at the journals.        